How to get rid of materials showing in the viewport when in solid mode
I'm having a bit of difficulty recreating your problem, but it's possible you have a type of viewport shading activated. Check the menu to the right inside your viewport (press 'n' if you don't see it) and look under the 'Shading' option. It should be located just beneath 'Display'. See if you have something like 'Textured Solids' checked. If you don't, an alternate way to address this problem is to apply a matcap, which you will find in shading, as well. Simply click on the picture after you tick the matcap box, and select the material you want.